Method for powering an electronically switched variable reluctance motor, and power supply circuit therefor

ABSTRACT

A method for powering an electronically switched variable reluctance motor (100) from a single-phase AC voltage source (17) is disclosed. Each phase of the motor is sequentially subjected to a magnetizing sequence, a demagnetizing sequence and an energy transfer to a storage capacitor (C) common to all the phases. Feedback sequences (R) in which the energy stored in the storage capacitor (C) is returned to the rectifier stage (15) are performed by controlling a chopping switch (TH) connected in series to the storage capacitor (C). The method includes a step of monitoring the energy returned to the rectifier stage (15) in order to correct instability in the rectified power supply current (Ia) relative to a reference current wave, and modulating the cyclical chopping ratio around a substantially constant mean value. The method is useful for powering household appliances.

This application is the National Stage of International ApplicationPCT/FR97/00109 under 35 USC §371, filed Jan. 20, 1997.

The present invention relates to a process for supplying a variablereluctance motor with electronic switching. It also relates to a supplycircuit for practicing this process.

The supply of DC current of a variable reluctance motor with electronicswitching from a mono-phase power supply generally uses a rectifierstage and a converter stage comprising electronic switching means thatis driven from instructions and positional information from detectors,to supply the different phases of this motor. The present inventionrelates more particularly to the case of variable reluctance motors withelectronic switching supplied from a capacitor of the C-Dump type withcurrent monitored as shown for example in the article "SwitchedReluctance Drives: New Aspects", IEEE Transactions on Power Electronics,Volume 5, No. 4, October 1990.

It is to be noted that in present processes for supplying a motor withelectronic switching, and particularly in the supply process used forthe C-Dump type converter, the rectified current delivered by therectifier stage is subject to disturbances with multiple frequencies ofthe frequency sector. These disturbances can lead to exceeding theharmonic values of current permitted by the European standard CEI 555-2.At present, this problem is handled by filtering the current deliveredby the rectifier stage so as to eliminate current harmonics higher thanthe frequency of the sector. There is inserted for example a passivefilter (based on filters L, C) or an active filter (of the PFC: "PowerFactor Correction" type), between the rectifier stage and the converterstage, as shown for example by the article "Effect of Power FactorCorrection Circuit on Switched Reluctance Motor Drives for Appliances"by R. KRISHNAN and S. LEE, APEC 94, Volume 1, Orlando, Feb. 13-17, 1994.The addition of a filter upstream of the converter stage however leadsto a significant increase in electronic costs when the powers to becontrolled exceeds several hundred Watts.

The object of the invention is to overcome these drawbacks by providinga process to supply a variable reluctance motor with electronicswitching, which permits satisfying both the requirements of low costand respect for the harmonic limit levels imposed by the mentionedEuropean standard.

These objects are achieved by a process for supplying a variablereluctance motor with electronic switching from a single phase ACvoltage source, comprising:

rectifying, in a rectifier stage, the voltage delivered by the singlephase AC source to supply a rectified voltage,

successively, for each of the phases of this motor:

a sequence of magnetization of said phase comprising an energy transferfrom the rectifier stage toward said phase to which is applied therectified voltage by closing switching means associated with said phase,

demagnetization sequence of said phase comprising a transfer of energyfrom said phase toward a storage condenser common to all the phases,while said switching means is opened, and

sequences of the return of stored energy in the capacity toward therectifier stage by control of chopping means in series with the storagecapacitor.

According to the invention, the process comprises monitoring the energyreturn toward the rectifier stage, so as to correct disturbances of therectified supply current.

It thus becomes possible to correct the disturbances of the supplycurrent and to be in conformity with the present requirements as tocompatibility of electromagnetic material, without having to usesystematically a passive or active filter upstream of the converterstage, and by using in a rational manner the energy resources normallypresent in the condenser of a converter of the C-Dump type. Thereresults a significant economic gain relative to known solutions.

There is meant more generally by disturbance of the current, anydeformation of the current wave relative to a reference current wave,this deformation being able to be induced by any physical phenomenonidentified or not, of an origin internal or external to the drive systemin question.

Disturbances of the rectified supply current can for example becorrected relative to a reference current wave. In a particularembodiment of the process according to the invention, the monitoring ofthe returned energy comprises a modulation of the cyclic chopping ratio.It can be provided for example that this cyclic ratio be modulated abouta substantially constant mean value. The mean value of the cyclic ratiofor chopping is thus preferably selected to be about 1/2 to maintain amean value of the voltage at the terminals of the storage capacitorabout twice the rectified supply voltage, so as to ensure correctdemagnetization of the phases of the motor.

In a preferred embodiment, this process comprises moreover a measurementof the rectified supply current, and a comparison of this measuredcurrent with the reference current wave, to determine the disturbancesto be corrected.

In a first embodiment of the process according to the invention, duringthe energy return sequence, an inductance serving as a magnetic bufferis interposed between the storage capacitor and the converter stage.

In a second embodiment of the process according to the invention, duringthe energy return sequence, the energy stored in the condenser isdirectly transferred toward the converter stage, and the process thuscomprises a filtering of the rectified supply current.

Moreover, the process according to the invention can preferably comprisealso a correction of the power factor of the motor/converter stageassembly as a function particularly of the measurement of the rectifiedsupply voltage and of the rectified supply current.

According to another aspect of the invention, there is proposed acircuit to supply a variable reluctance motor with electronic switching,comprising:

a rectifier stage to deliver from a single phase AC voltage source, arectified voltage,

a converter stage to supply each of the phases of said motor, thisconverter stage comprising, for each phase, a phase switching meanscontrolled to magnetize in a predetermined time window, said phase byuse of the rectified voltage, these switching means being moreovercontrolled to demagnetize said phase by transfer of magnetic energystored in said phase toward a storage capacitor common to all the phasesof the motor,

means to return the energy stored in the storage capacitor toward therectifier stage, comprising chopping means in series with the storagecapacitor, and

means to control the phase switching means and the chopping means, as afunction particularly of operating instructions and measurements ofelectrical magnitude, particularly the rectified supply current and thevoltage at the terminals of the storage capacitor.

According to the invention, the control means are arranged to controlaccording to a variable cyclic ratio the chopping means so as to correctthe disturbances observed in the rectified supply current.

In a first embodiment of a circuit according to the invention, thelatter comprises moreover an inductance disposed between the choppingmeans and the converter stage and serving as a magnetic buffer.

In a second embodiment of a circuit according to the invention, thelatter comprises moreover a filter stage disposed between the rectifierstage and the converter stage.

Other features and advantages of the invention will become apparent fromthe following description. In the accompanying drawings, given by way ofnon-limiting example:

FIG. 1 is a synoptic diagram of a motor system with variable reluctancefor practicing the process and the supply circuit according to theinvention;

FIG. 2 shows a first embodiment of a supply circuit according to theinvention;

FIGS. 3A, 3B and 3C show schematically the energy transfers in thecircuit shown in FIG. 2, respectively in the sequences of magnetization,demagnetization and energy return;

FIG. 4 shows a second embodiment of a supply circuit according to theinvention;

FIGS. 5A, 5B and 5C show schematically the energy transfers in thecircuit shown in FIG. 4, respectively in the sequences of magnetization,demagnetization and energy return; and

FIG. 6 is a modification of the circuit shown in FIG. 4.

There will now be described, with reference to the foregoing figures,the general structure of a supply circuit according to the invention andtwo examples of embodiment, at the same time as the supply process usedin these circuits.

A variable speed drive system 1 using the process according to theinvention comprises, with reference to FIG. 1, a rectifier stage 15connected to a source of single phase AC voltage 17, for example thesector, a converter stage 3 supplying the three phases of a variablereluctance motor with electronic switching 100, a stage 4 for energyreturn, and a control device 23 receiving at its input instructions,data 25 as to angular position of the rotor of the motor andmeasurements 24, 26 of the physical size associated with the system, andgenerating control orders 28, 27 for electronic switches of theconverter stage 3 and a chopping switch TH of the energy return stage 4.

In a first embodiment of the supply circuit 2 shown in FIG. 2, theconverter stage 3 is directly connected to the output of the rectifierstage 15 without use of a filter, and an inductance L is disposedbetween the common high point of the converter stage 3 and the energyreturn stage 4. The rectifier stage 15 can have the conventionalstructure of a four-diode bridge. The converter stage 3 is of the C-Dumptype. It comprises a high line 21 and a low line 22 connectedrespectively to the output terminals of the rectifier stage 15, andthree branches connected in parallel each comprising a power switch T1,T2, T3, for example a transistor IGBT, connected on the one hand to oneterminal of a phase of the motor 100, and on the other hand to the lowline 22. The three phases Lph1, Lph2, Lph3 are connected in star fashionand their common connection is connected to the high line of theconverter stage 3. Moreover, a free wheel diode 18 is conventionallyprovided between the high and low lines 21, 22 to ensure continuity ofcurrent drawn by the converter stage 3 and thus to avoid overvoltage. Ateach connection point of a switch T1, T2, T3 and of the phase Lph1,Lph2, Lph3 which is associated with it is connected the anode ofdemagnetization diode D1, D2, D3. The energy return stage 4 comprises inseries a storage capacitor C, a chopping switch TH and a diode 16. Thecathodes of the three demagnetization diodes D1, D2, D3 are connected tothe connection point between the storage capacitor C and the choppingswitch TH which can for example be an IGBT transistor. A control device23 for the system 1 comprises a control module 10 for the choppingswitch TH and a control module 35 for the three phase switches T1, T2,T3. The chopping control module 10 receives a measurement 11 of therectified supply voltage, a measurement 12 of the voltage at the twoterminals of the storage capacitor C, a measurement 13 of the rectifiedsupply current Ia supplied to the terminals of a shunt 29, and in turngenerates control orders for the chopping switch TH and control orders14 for the phase control module 35. This latter also receives positiondata as to the rotor of the motor, supplied for example by an angularposition detector, and operating instructions, for example a speedinstruction, and generates in turn control orders for the phase switchesT1, T2, T3.

There will now be described the operation of this supply circuitaccording to the invention, as well as the energy transfers carried outin the course of the different sequences of the process according to theinvention, with reference to FIGS. 2 and 3A to 3C. It should be notedthat, for reasons of simplicity, the energy flow in FIGS. 3A to 3C isshown only for a single phase.

The rectifier stage 15 delivers a rectified voltage in the form ofdouble alternating arches. The rectified supply current Ia withdrawn bythis rectifier stage 15 is a function of the different phase switchingswhich are carried out with variable frequency. The object of the processaccording to the invention is to approach a rectified supply currentwhich will have the hypothetical wave form of that of the rectifiedvoltage. Each phase Lph1, Lph2, Lph3 of the motor 100 is subjected tosuccessive magnetization sequences (M) and demagnetization sequences (D)at the rhythm of the phase switching.

It should be noted that, according to the control modes used, there canbe simultaneously a magnetization sequence of one phase and ademagnetization sequence of another phase. More generally, the supplyprocess according to the invention can be used no matter what the levelof superposition between respective sequences of magnetization anddemagnetization concerning the different phases of the motor.

In the course of a magnetization sequence (M) of a given phase of themotor, the electrical energy is transferred from the source 17, via therectifier stage 15 and a branch of the converter stage 3, toward a motorphase in which it is partially converted to the form of mechanicalenergy and as to another portion stored in the form of magnetic energyin the motor (FIG. 3A). The supply current Ia is thus equal to the sumof the currents in the three phases of the motor. When the correspondingphase switch is again blocked, there then begins a demagnetizationsequence (D) in the course of which the magnetic energy stored in themotor 100 is transferred via the demagnetization diode into the storagecapacitor in the form of electrostatic energy (FIG. 3B). In the courseof the demagnetization sequence (D), the rectifier stage 4 continues tosupply electrical energy to the system. This demagnetization is renderedpossible by the fact that the voltage at the terminals of the storagecapacitor is maintained at a mean voltage about double the rectifiedsupply voltage. The chopping stage 4 is controlled in passing mode onthe one hand, when the voltage at the terminals of the storage capacitorexceeds a safety limit value, and on the other hand, when the measuredsupply current becomes greater than a standard connected to a referencecurrent wave. Chopping is thus carried out at a chopping frequency andat a variable cyclic ratio. In the course of an energy return sequence(R), a fraction of the energy stored in the storage capacitor C istransferred via the inductance L toward the rectifier stage 15, whilstthe phase switch is thus in passing mode. The supply current Ia is thusequal to the difference between the sum ΣIph of the currents in themotor phases and the current Ic delivered by the storage capacitor C.When the measured supply current becomes less than the reference wavecurrent, the chopping switch is thus switched to blocking mode and theenergy return sequence is terminated. It can thus be provided that thecontrol of the chopping switch TH will not be effective except when thevoltage measured at the terminals of the storage capacitor C is greaterthan the measured rectified supply voltage.

An essential difference of the supply process according to theinvention, compared to conventional processes, resides in the fact thatthe disturbances of the supply current are corrected without theaddition of supplemental power components, by applying a variable cyclicratio regime to the chopping switch of the C-Dump mounting.

There will now be described, with reference to FIG. 4, a secondembodiment of the supply process according to the invention and anexample of an embodiment of a corresponding supply circuit. In FIGS. 4and 5A to 5C, the same reference numerals are applied to identicalcomponents and modules already described with reference to FIGS. 2 and3A to 3C. It should be noted that, for reasons of simplification, theenergy flow in FIGS. 5A to 5C is shown only for a single phase.

The supply circuit 6 according to the invention comprises a rectifierstage 15, a filtering stage F, a converter stage 3 connected to thethree phases of a variable reluctance motor with electronic switching100, and an energy return stage 4. The filtering stage F comprises forexample a passive filter comprising conventionally an inductance L1 anda capacitor C1, and a diode D' to isolate on the one hand the filteringstage and on the other hand the converter stage in the course of thereturn sequence. The energy return stage 4 is, in this particularembodiment, directly connected to the converter stage 3 without theaddition of inductance nor of a free wheel diode in parallel to thechopping stage.

There will now be described the operation of the supply circuitaccording to the invention, at the same time as the energy transfer iscarried out in the course of the different sequences of the processaccording to the invention, with reference to FIGS. 4 and 5A to 5C. Themagnetization and demagnetization sequences (FIGS. 5A and SB) aresimilar to the sequences described for the first embodiment, except forthe fact that the filtering stage F is contributory. In the energyreturn sequences (R), when the chopping switch TH is controlled inpassing mode, the voltage at the terminals of the storage capacitor C isdirectly applied to the high line 21. As this voltage is thennecessarily higher than the rectified supply voltage, the diode D' isblocked and the supply current Ia will then decrease. As soon as ablocking order is applied to the chopping switch TH, the diode D' againbecomes passing and the rectified supply current then tends to increase.It has been noted that a filter of modest size suffices. In the courseof the energy return sequence, the supply current Ia is equal to thecurrent Ic1 absorbed by the filtering capacitor C1, whilst the currentIc supplied by the capacitor C is equal to the magnetization currentflowing in one or several phases.

The chopping control modules 10, 20 include preferably a power factorcorrection circuit (PFC) which delivers a signal indicative of the weakor strong character of the supply current, from current referenceinformation (obtained in practice from the measurement of the rectifiedsupply voltage) and of the information as to current supply effectivelymeasured.

FIG. 6 shows a modification of the supply circuit of FIG. 4, in whichthere are applied the same reference numerals to the identicalcomponents of modules already described with reference to FIG. 4. Thismodification is different from the supply circuit shown in FIG. 4 solelyby the fact that the filtering stage F comprises, in addition to theinductance L1 disposed in the high line 21 of the circuit, a diode D"disposed between the high line 21 and low line 22 of the circuit, theanode of this diode being connected to the low line 22. Thus, thisfiltering stage F has the advantage of being particularly simplified,from which follows a reduction of its cost of production. The operationof this modification of the supply circuit, as well as the energytransfers carried out in the course of the different sequences of theprocess according to the invention, are similar to those described withreference to FIGS. 5A to 5C.

Of course the invention is not limited to the examples which have beendescribed and of which numerous variations may be realized withoutdeparting from the scope of the invention. Thus, the number of motorphases could be as desired. The phase in chopping switches could betransistors of any technology, for example bipolar transistors, MOSFETor IGBT transistors. The control modules could be in the form ofhardwired or programmed logic and could include a microprocessor.Moreover, the supply circuits according to the invention could beintegrated in the form of ASIC circuits or even could be embodied withseparate components.

I claim:
 1. Process to supply a variable reluctance motor withelectronic switching (100) from a single phase AC voltage source (17),comprising:rectifying, in a rectifier stage (15), the voltage deliveredby the single phase AC source (17) to supply a rectified voltage,successively, for each of the phases (Lph1, Lph2, Lph3) of this motor(100):a magnetization sequence (M) of said phase (Lph1, Lph2, Lph3)comprising a transfer of energy from the rectifier stage (15) towardsaid phase (Lph1, Lph2, Lph3) to which is applied the rectified voltageby closing switching means (T1, T2, T3) associated with said phase(Lph1, Lph2, Lph3), a demagnetization sequence (D) of said phase (Lph1,Lph2, Lph3) comprising a transfer of energy from said phase (Lph1, Lph2,Lph3) toward a storage capacitor (C) common to all said phases (Lph1,Lph2, Lph3), upon opening said switching means (T1, T2, T3), and returnsequences (R) of the energy stored in the storage capacitor (C) towardthe rectifier stage (15) by control of chopping means (TH) in serieswith the storage capacitor (C), characterized in that it comprisesmonitoring the energy returned to the rectifier stage (15), so as tocorrect disturbances of the rectified supply current (Ia).
 2. Processaccording to claim 1, characterized in that the disturbances of therectified supply current (Ia) are corrected relative to a referencecurrent wave.
 3. Process according to claim 2, characterized in that itcomprises also a measurement of the rectified supply current (Ia), and acomparison of this measured current with the reference current wave, todetermine the disturbances to be corrected.
 4. Process according to oneof claims 1, characterized in that the monitoring of the returned energycomprises a modulation of the cyclic chopping ratio.
 5. Processaccording to claim 4, characterized in that the cyclic chopping ratio ismodulated about a substantially constant mean value.
 6. Processaccording to claim 5, characterized in that the mean value of the cyclicchopping ratio is about 1/2.
 7. Process according to claim 1characterized in that, during the energy return sequence (R), aninductance (L) serving as a magnetic buffer is interposed between thestorage capacitor (C) and the rectifier stage (15).
 8. Process accordingto claim 1, characterized in that, during the energy return sequence(R), the energy stored in the storage capacitor (C) is directlytransferred to the rectifier stage (15), and in that it comprises also afiltering of the rectified supply current (Ia).
 9. Process according toclaim 1, characterized in that it also comprises a correction of thepower factor of the motor as a function of the measurement of therectified supply voltage and of the rectified supply current. 10.Circuit to supply a variable reluctance motor with electronic switching(100), comprising:a rectifier stage (15) to deliver from a single phaseAC voltage source (17) a rectified voltage, a converter stage (3) tosupply each of the phases (Lph1, Lph2, Lph3) of said motor (100), thisconverter stage (3) comprising, for each phase (Lph1, Lph2, Lph3), phaseswitching means (T1, T2, T3) controlled to magnetize in a predeterminedtime window of said phase (Lph1, Lph2, Lph3) by use of the rectifiedvoltage, these phase switching means (T1, T2, T3) being moreovercontrolled to demagnetize said phase (Lph1, Lph2, Lph3) by transfer ofthe magnetic energy stored into said phase (Lph1, Lph2, Lph3) toward astorage capacitor (C) common to all the phases of the motor (100), means(4) to return the energy stored in the storage capacitor (C) toward therectifier stage (15), comprising chopping means (TH), and means (23, 10,20, 35) to control the phase switching means (T1, T2, T3) and thechopping means (TH) as a function of operating instructions andmeasurements of the electrical quantities, namely the rectified supplycurrent and the voltage at the terminals of the storage capacitor;characterized in that the control means (23, 10, 20, 35) are arranged tocontrol according to a variable cyclical ratio of the chopping means(TH) so as to correct the disturbances observed in the rectified supplycurrent (Ia).
 11. Circuit according to claim 10, characterized in thatit comprises moreover an inductance (L) disposed between the choppingmeans (4) and the converter stage (3) and serving as a magnetic buffer.12. Circuit according to claim 10, characterized in that the choppingmeans (4) are directly connected to the converter stage (3) and in thatit comprises moreover a filtering stage (F) disposed between therectifier stage (15) and the converter stage (3).
 13. Circuit accordingto claim 12, characterized in that the filtering stage (F) comprises aninductance (Li) disposed in the high line (21) of the rectifier stage(15), and a diode (DI) disposed between the high line (21) and low line(22) of said rectifier stage, the anode of the diode (D") beingconnected to said low line (22).
 14. Circuit according to claim 10,characterized in that the control means (23) comprise means to correctthe power factor of the motor.